1. Field of the Invention
The present invention relates to an optical scanning apparatus for scanning a surface with a deflected optical beam, such as a laser light, and to a recording apparatus using the optical scanning apparatus.
2. Description of the Related Arts
Optical scanners for scanning a surface in two dimensional directions with a spot of light formed on the surface from a repetitively deflected optical beam such as a laser have been widely used in printers such as laser printers.
A polygon mirror or a galvanomirror is usually used as deflecting means of the optical scanner. In the deflecting means, an error of the position of a scanning line readily occurs due to a leaning error of the reflecting surface. In order to correct such an error, a cylindrical lens for converging an incident beam once in a subscanning direction to focus in the vicinity of the reflecting surface of the deflecting means is provided to give a conjugate relationship between the reflecting surface and the scanned surface (i.e. a recording surface).
The optical system of such an optical scanning apparatus should be capable of moving the cylindrical lens in an optical axis direction in order to absorb manufacturing errors of the lenses and the housing so as to converge the optical beam to form the spot precisely on the scanned surface, to absorb the variations in astigmatism in case where a semiconductor laser is used, or to adjust the spot diameter in the subscanning direction.
An approach to move the cylindrical lens in the subscanning direction has been known for adjusting the axes of the cylindrical lens and the optical system in the subscanning direction.
It has been demanded that the recording apparatus using the optical scanning apparatus has a capability of changing the recording density at need. It is necessary that the effective diameter of a spot of the optical beam formed on a recording surface can be changed to an optimum value for video recording in order to maintain the best quality of the recorded video image without spaces between recording spots when the recording density is changed. It is possible to adjust the recording spot size in the main scanning direction by changing the modulation (switching) time of the optical beam. Accordingly, it is common that the optical system is arranged so that only the diameter of the spot in the subscanning direction is variable. An approach of changing the spot diameter in the subscanning direction may be an approach of changing power of the cylindrical lens, an approach of adjusting an opening diameter of an aperture to change the spot diameter of the optical beam incident upon the cylindrical lens, and an approach of intentionally defocusing the spot in the subscanning direction. An arrangement for adjusting a cylindrical lens as the third approach is disclosed in JP-A-57-144517.
Such an optical system is a so-called toric lens system in which the powers in the main scanning direction and in the subscanning direction are different from each other. Therefore, the axes of a plurality of lenses should be aligned with a high precision. It is generally necessary to carry out rotation of the lenses around the optical axis with a high precision. Particularly, since the cylindrical lenses are small in size and a space for mounting the lenses is narrow, the precision required for mounting them is considerably high. In other words, an extremely fine alignment around the optical axis is required for the cylindrical lenses. Therefore, an adjusting and holding mechanism with a high precision is needed for adjusting the cylindrical lenses in the optical axis to change the spot diameter or for adjusting the cylindrical lenses to align the axes in the scanning directions.
The approach to change power of cylindrical lenses requires lens exchange or cylindrical lenses as zoom lenses which the zooming in one direction. The former approach to exchange the lenses requires readjustment on exchange and the latter requires an expensive zoom lens mechanism.
Although the approach to adjust the diameter of the incident optical beam has an advantage that the geometrical shape of the image is not changed, the approach has a defect that the efficiency of the optical path changes according to change in diameter of the beam. In case a semiconductor laser is used as the light source, there is a problem that the amount of change in the efficiency of the optical path or in the spot diameter is not constant due to variations of the emission angle of the semiconductor laser.
Takaoka et al. disclose in JP-A-52-84748 a method of finely adjusting position of convergence of a laser light by controlling an effective thickness of flat plate glasses combined with wedge type prisms. This method cannot adjust the optical axis.
Caviglia et al. and in U.S. Pat. No. 4,900,120 disclose a coupling device for adjusting direction of an optical axis of a collimated optical beam by rotation of flat plate glasses but they do not consider the adjustment of the focal position.
Morimoto et al. in U.S. Pat. No. 4,850,686 disclose a method of adjusting the axis of light rays by the combination of prisms rather than flat plate glasses. The spot size cannot be adjusted by Morimoto method.